]> CryptoNext Security

daniel.vangeest@cryptonext-security.com

MTG AG

falko.strenzke@mtg.de

Security Limited Additional Mechanisms for PKIX and SMIME Cryptographic Message Syntax CMS Signed Attributes signedAttrs SignedData The Cryptographic Message Syntax (CMS) has different signature verification behaviour based on whether signed attributes are present or not. This results in a potential existential forgery vulnerability in CMS and protocols which use CMS. This document describes the vulnerability and lists mitigations and best practices to avoid it. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The Cryptographic Message Syntax (CMS) signed-data content type allows any number of signers in parallel to sign any type of content. CMS gives a signer two options when generating a signature on some content:

• Generate a signature on the whole content; or
• Compute a hash over the content, place this hash in the message-digest attribute in the SignedAttributes type, and generate a signature on the SignedAttributes. The SignedAttributes type is placed in the signedAttrs field of the SignedData type.

The resulting signature does not commit to the presence of the SignedAttributes type, allowing an attacker to influence verification behaviour. An attacker can perform two different types of attacks:

1. Take an arbitrary CMS signed message M which was originally signed with SignedAttributes present and rearrange the structure such that the SignedAttributes field is absent and the original DER-encoded SignedAttributes appears as an encapsulated or detached content of type id-data, thereby crafting a new structure M' that was never explicitly signed by the signer. M' has the DER-encoded SignedAttributes of the original message as its content and verifies correctly against the original signature of M.
2. Let the signer sign a message of the attacker's choice without SignedAttributes. The attacker chooses this message to be a valid DER-encoding of a SignedAttributes object. The attacker can then add this encoded SignedAttributes object to the signed message and change the signed message to the one that was used to create the messageDigest attribute within the SignedAttributes. The signature created by the signer is valid for this arbitrary attacker-chosen message.

This vulnerability was presented by Falko Strenzke to the LAMPS working group at IETF 121 and is detailed in . states:

• signedAttrs is a collection of attributes that are signed. The field is optional, but it MUST be present if the content type of the EncapsulatedContentInfo value being signed is not id-data.

Thus, if a verifier accepts a content type of id-data in the EncapsulatedContentInfo type when used in SignedData, then a SignerInfo within the SignedData may or may not contain a signedAttrs field and will be vulnerable to this attack. On the other hand, if the verifier doesn't accept a content type of id-data, the sender always adds the signedAttrs field, and the recipient verifies that signedAttrs is present, the attack will not succeed. The limited flexibility of either the signed or the forged message in either attack variant may mean the attacks are only narrowly applicable. Nevertheless, due to the wide deployment of the affected protocols and the use of CMS in many proprietary systems, the attacks cannot be entirely disregarded. As a mitigation, this document defines the new mimeData content type to be used in new uses of the CMS SignedData type when the encapsulated content is MIME encoded and thus avoid the use of the id-data content type. This document further describes best practices and mitigations that can also be applied to those protocols or systems that continue to use the content type id-data.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

mimeData Content Type The following object identifier identifies the mimeData content type: The mimeData content type is intended as a replacement for the id-data content type in new uses of the CMS SignedData type where the content is MIME encoded.

Best Practices This section describes the best practices to avoid the vulnerability at the time of writing. New uses of the CMS SignedData MUST NOT use the id-data EncapsulatedContentInfo content type. If the new content is MIME encoded, the mimeData content type SHOULD be used.

Existing Uses of id-data When a protocol which uses the id-data EncapsulatedContentInfo content type within SignedData is updated, it SHOULD deprecate the use of id-data and use a different (new or existing) identifier. A partial list of such identifiers is found in the "CMS Inner Content Types" IANA subregistry within the "Media Type Sub-Parameter Registries". If the existing content is MIME encoded, the mimeData content type SHOULD be used. Updated protocols that do not deprecate the use of id-data SHOULD provide a rationale for not doing so. Note that accepting the content type id-data during verification is sufficient for a vulnerability to surface. Hence the measures described in must be adhered to. When a protocol uses the id-data EncapsulatedContentInfo content type within SignedData, it SHOULD specify that the signedAttrs field is either always required or always forbidden. If a protocol makes such a requirement, a recipient MUST check whether the signedAttrs field is present or absent as specified by the protocol, and fail processing if the appropriate condition is not met.

Recipient Verification When a recipient receives a CMS SignedData, it SHOULD be checked that the EncapsulatedContentInfo content type value is the one expected by the protocol and fail processing if it is not. As specified in , a SignerInfo signedAttrs field MUST be present if the content type of the EncapsulatedContentInfo value being signed is not id-data. To avoid the attack described in , when a recipient receives a CMS SignedData and the EncapsulatedContentInfo content type is not id-data, it SHOULD verify both that the expected content type was received and that each SignerInfo contains the signedAttrs field, and fail processing if either of these conditions is not met.

Mitigations This section describes mitigations for cases where the best practices given above cannot be applied. When the id-data EncapsulatedContentInfo content type is used, the following mitigations MAY be applied to protect against the vulnerability described in .

Recipient Detection This mitigation is performed by a recipient when processing SignedData. If signedAttrs is not present, check if the encapsulated or detached content is a valid DER-encoded SignedAttributes structure and fail if it is. The mandatory contentType and messageDigest attributes, with their respective OIDs, should give a low probability of a legitimate message which happens to look like a DER-encoded SignedAttributes structure being flagged. However, a malicious party could intentionally present messages for signing that are detected by the countermeasure and thus introduce errors into the application processing that might be hard to trace for a non-expert.

Sender Detection This mitigation is performed by a sender who signs data received from a 3rd party (potentially an attacker). If the sender is signing 3rd party content and will not be setting the signedAttrs field, check that the content is not a DER-encoded SignedAttributes structure, and fail if it is. Note that also in this case, a malicious party could intentionally present messages that trigger this countermeasure and thereby trigger hard-to-trace errors during the signing process.

Security Considerations The vulnerability is not present in systems where the use of signedAttrs is mandatory, as long as recipients enforce the use of signedAttrs. Some examples where the use of signedAttrs is mandatory are SCEP, Certificate Transparency, RFC 4018 firmware update, German Smart Metering CMS data format. Any protocol that uses an EncapsulatedContentInfo content type other than id-data is required to use signed attributes. However, this security relies on a correct implementation of the verification routine that ensures the correct content type and presence of signedAttrs. When the message is signed and then encrypted, though in the general case this will make it difficult for the attacker to learn the signature, the vulnerability might still be present if mitigations are not applied:

• Signing and encryption might not be done on the same endpoints, in which case an attacker between the endpoints might be able to learn the signature for which it could remove or add the signedAttrs.
• IND-CPA encryption does not give theoretical guarantees against an active attacker and thus does not guarantee that an attacker cannot rearrange the structure.

Conceivably vulnerable systems:

• Unencrypted firmware update denial of service
  • Secure firmware updates often use signatures without encryption. If the forged message can bring a device, due to lack of robustness in the parser implementation, into an error state, this may lead to a denial of service vulnerability. The possibility of creating a targeted exploit can be excluded with great certainty in this case due to the lack of control the attacker has over the forged message.
• Dense message space
  • If a protocol has a dense message space, i.e. a high probability that the forged message represents a valid command or the beginning of a valid command, then, especially if the parser is permissive with respect to trailing data, there is a risk that the message is accepted as valid. This requires a protocol where messages are signed but not encrypted.
• Signing unstructured data
  • Protocols that sign unencrypted unstructured messages, e.g. tokens, might be affected in that the signature of one token might result in the corresponding forged message being another valid token.
• External signatures over unstructured data
  • The probably strongest affected class of systems would be one that uses external signatures, i.e. CMS signatures with absent content (that may be transmitted encrypted separately) over unstructured data, e.g. a token of variable length. In that case the attacker could create a signed data object for a known secret.
• Systems with permissive parsers
  • In addition to potential issues where the protocol parser is permissive (e.g. with respect to trailing space), if the CMS parser is permissive (e.g. allows non-protocol content types, or allows missing signedAttrs with content types other than id-data) then this could result in accepting invalid messages.

Further note that it is generally not good security behaviour to sign data received from a 3rd party without first verifying that data. describes just one verification step that can be performed, specific to the vulnerability described in .

ASN.1 Module MimeData-2026 { iso(1) member-body(2) usa(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) modules(0) id-mod-mime-data-2026(TBD1) } DEFINITIONS EXPLICIT TAGS ::= BEGIN IMPORTS CONTENT-TYPE FROM CryptographicMessageSyntax-2010 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } ; id-mime-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) TBD2 } ct-MimeData CONTENT-TYPE ::= { IDENTIFIED BY id-mime-data } END ]]>

IANA Considerations In the "SMI Security for S/MIME Module Identifier" registry, create a new entry to point to this document.

Decimal	Description	Reference
TBD1	id-mod-mime-data-2026	[[This Document]]

In the "SMI Security for S/MIME Content Types" registry, add a new entry for id-mime-data that points to this document.

Decimal	Description	Reference
TBD2	id-mime-data	[[This Document]]

In the table "CMS Inner Content Types" add a new entry:

Name	Object Identifier	Reference
mimeData	1.2.840.113549.1.9.16.1.TBD2	[[This Document]]

References Normative References This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document specifies the Simple Certificate Enrolment Protocol (SCEP), a PKI protocol that leverages existing technology by using Cryptographic Message Syntax (CMS, formerly known as PKCS #7) and PKCS #10 over HTTP. SCEP is the evolution of the enrolment protocol sponsored by Cisco Systems, which enjoys wide support in both client and server implementations, as well as being relied upon by numerous other industry standards that work with certificates. This document presents a technique to securely provision a networking device when it is booting in a factory-default state. Variations in the solution enable it to be used on both public and private networks. The provisioning steps are able to update the boot image, commit an initial configuration, and execute arbitrary scripts to address auxiliary needs. The updated device is subsequently able to establish secure connections with other systems. For instance, a device may establish NETCONF (RFC 6241) and/or RESTCONF (RFC 8040) connections with deployment-specific network management systems. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. This document defines the bundle security protocol, which provides data integrity and confidentiality services for the Bundle Protocol. Separate capabilities are provided to protect the bundle payload and additional data that may be included within the bundle. We also describe various security considerations including some policy options. This document is a product of the Delay-Tolerant Networking Research Group and has been reviewed by that group. No objections to its publication as an RFC were raised. This document defines an Experimental Protocol for the Internet community. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.2. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 3851. [STANDARDS-TRACK] This document describes a file format for the storage of flow data based upon the IP Flow Information Export (IPFIX) protocol. It proposes a set of requirements for flat-file, binary flow data file formats, then specifies the IPFIX File format to meet these requirements based upon IPFIX Messages. This IPFIX File format is designed to facilitate interoperability and reusability among a wide variety of flow storage, processing, and analysis tools. [STANDARDS TRACK] This document defines a practical architecture and protocols for offering privacy for a user who requests and uses an X.509 certificate containing a pseudonym, while still retaining the ability to map such a certificate to the real user who requested it. The architecture is compatible with IETF certificate request formats such as PKCS10 (RFC 2986) and CMC (RFC 5272). The architecture separates the authorities involved in issuing a certificate: one for verifying ownership of a private key (Blind Issuer) and the other for validating the contents of a certificate (Anonymity Issuer). The end entity (EE) certificates issued under this model are called Traceable Anonymous Certificates (TACs). This memo defines an Experimental Protocol for the Internet community. This document defines the format of an electronic signature that can remain valid over long periods. This includes evidence as to its validity even if the signer or verifying party later attempts to deny (i.e., repudiates) the validity of the signature. The format can be considered as an extension to RFC 3852 and RFC 2634, where, when appropriate, additional signed and unsigned attributes have been defined. The contents of this Informational RFC amount to a transposition of the ETSI Technical Specification (TS) 101 733 V.1.7.4 (CMS Advanced Electronic Signatures -- CAdES) and is technically equivalent to it. The technical contents of this specification are maintained by ETSI. The ETSI TS and further updates are available free of charge at: http://www.etsi.org/WebSite/Standards/StandardsDownload.aspx This memo provides information for the Internet community. This memo updates the ODETTE File Transfer Protocol, an established file transfer protocol facilitating electronic data interchange of business data between trading partners, to version 2. The protocol now supports secure and authenticated communication over the Internet using Transport Layer Security, provides file encryption, signing, and compression using Cryptographic Message Syntax, and provides signed receipts for the acknowledgement of received files. The protocol supports both direct peer-to-peer communication and indirect communication via a Value Added Network and may be used with TCP/IP, X.25, and ISDN-based networks. This memo provides information for the Internet community. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.1. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 2633. [STANDARDS-TRACK] This document defines the format of an electronic signature that can remain valid over long periods. This includes evidence as to its validity even if the signer or verifying party later attempts to deny (i.e., repudiates the validity of the signature). This memo provides information for the Internet community. This document describes a protocol for adding cryptographic signature and encryption services to MIME data. [STANDARDS-TRACK]

RFCs Using the id-data EncapsulatedContentInfo Content Type This appendix lists RFCs which use the id-data content type in EncapsulatedContentInfo. It is a best-effort list by the authors at time of authorship. The list can be used as a starting point to determine if any of BCPs in this document can be applied. The following table summarizes the RFCs' usages of signed attributes. RFCs using id-data

RFC	Signed Attributes Usage
	Requires the used of signed attributes
	Says nothing about signed attributes
	RECOMMENDS signed attributes
	Forbids signed attributes
	RECOMMENDS signed attributes
	Says nothing about signed attributes
	Forbids signed attributes
	Requires signed attributes
	Says nothing about signed attributes
	RECOMMENDS signed attributes
	Requires signed attributes
	RECOMMENDS signed attributes

An RFC requiring or forbidding signed attributes does not necessarily mean that a recipient will enforce this requirement when verifying, their CMS implementation may simply process the message whether or not signed attributes are present. If one of the signed attributes is necessary for the recipient to successfully verify the signature or to successfully process the CMS data then the vulnerability will not apply; at least not when assuming the signer is well-behaved and always signs with signed attributes present in accordance with the applicable specification.

RFC 8894 Simple Certificate Enrolment Protocol Figure 6 in specifies id-data as the EncapsulatedContentInfo content type, and shows the use of signedAttrs. The document itself never refers to signed attributes, but instead to authenticated attributes and an authenticatedAttributes type. Errata ID 8247 clarifies that it should be "signed attributes" and "signedAttrs". Since SCEP requires the use of signedAttrs with the id-data EncapsulatedContentInfo content type, and the recipient must process at least some of the signed attributes, it is not affected by the vulnerability.

RFC 8572 Secure Zero Touch Provisioning (SZTP) allows the use of the id-data content type, although it also defines more specific content types. It does not say anything about signed attributes.

S/MIME RFCs , , , and require the use of the id-data EncapsulatedContentInfo content type. says:

• Receiving agents MUST be able to handle zero or one instance of each of the signed attributes listed here. Sending agents SHOULD generate one instance of each of the following signed attributes in each S/MIME message:

and

• Sending agents SHOULD generate one instance of the signingCertificate or signingCertificateV2 signed attribute in each SignerInfo structure.

So the use of signed attributes is not an absolute requirement.

RFC 6257 Bundle Security Protocol Specification says:

• In all cases where we use CMS, implementations SHOULD NOT include additional attributes whether signed or unsigned, authenticated or unauthenticated.

It does not specify what the behaviour should be if signed attributes are found by the receiver.

RFC 5655 IP Flow Information Export (IPFIX) is a file format that uses CMS for detached signatures. It says nothing about the use of signed attributes.

RFC 5636 Traceable Anonymous Certificate says:

• The signedAttr element MUST be omitted.

It does not specify what the behaviour should be if signed attributes are found by the receiver.

RFC 5126 CMS Advanced Electronic Signatures (CAdES) specifies mandatory signed attributes. One of the signed attributes is used to determine which certificate is used to verify the signature, so CaDES is not affected by the vulnerability.

RFC 5024 ODETTE File Transfer Protocol 2 uses the id-data EncapsulatedContentInfo content type and says nothing about signed attributes.

RFC 3126 Electronic Signature Formats for long term electronic signatures requires the MessageDigest attribute, which is a signed attribute.

Acknowledgments The authors would like to thank Russ Housley, Carl Wallace, and John Preuß Mattsson for their valuable feedback on this document.